The invention relates to electronic transformers, in particular those which are suitable for operating halogen incandescent lamps. The invention improves the immunity of the electronic transformer to high-voltage pulses.
Electronic transformers generally have the following design: the mains voltage supplied from the mains is firstly rectified. The rectified mains voltage supplies energy for an inverter. The inverter outputs a high-frequency voltage which can be transformed to the desired voltage with the aid of an output transformer. High frequency is understood in this context as frequencies which are substantially above the mains frequency. The prior art in this case comprises a frequency band from a plurality of kilohertz to over 1 megahertz. The application of electronic transformers for operating halogen incandescent lamps is very widespread. It is not necessary when operating these lamps to supply a constant voltage to the lamp. Because of the thermal inertia of the incandescent filament of these lamps, the voltage present across the lamp may be amplitude-modulated with the mains frequency. The capacitor, required for a constant output voltage, for energy storage can therefore be eliminated. However, this results in the following disadvantage: in accordance with regulation IEC 1047, an electronic transformer for halogen incandescent lamps must be immune to high-voltage pulses which are superimposed on the mains voltage and have an amplitude of 1000V. (The term surge resistance has become established for this in the literature, and will be used below.) It has emerged in practice that it is even desirable for the surge resistance to exceed the measure demanded in the standards. In principle, it is possible to use voltage-dependent resistors or semiconductors such as, for example, varistors or suppressor diodes against high-voltage pulses. However, it has emerged that these measures alone do not suffice to protect an electronic transformer against high-voltage pulses occurring in practice. Rather, todate it has been necessary to overdimension radio interference suppression filters and power transistors in order to achieve an adequate surge resistance.
It is an object of the present invention to provide an electronic transformer which in a cost effective fashion and with low outlay provides a surge resistance which satisfies at least the requirements of the relevant standards. The invention proceeds from an electronic transformer with a self-excited half-bridge inverter which includes a half-bridge with a series circuit of two electronic switches. Self-excited half-bridge inverters generally require a starting circuit which triggers a natural oscillation of the half-bridge inverter. This mostly occurs by virtue of the fact that an electronic switch of the half-bridge is driven for a short time by a starting capacitor. During the self-excited oscillation, it is necessary to suppress a starting operation by means of a blocking device, since otherwise it can occur that both switches of the half-bridge switch on simultaneously, and this leads to what are termed cross currents which destroy the half-bridge in a short time. The blocking device generally includes an electronic discharging switch which discharges the starting capacitor in a clock fashion through the half-bridge. In the case of electronic transformers which correspond to the prior art, the starting capacitor is always discharged whenever that switch of the half-bridge is conducting which is driven by the starting circuit upon triggering of the natural oscillation of the half-bridge. Precisely for this reason, however, the electronic transformer is sensitive to high-voltage pulses. Specifically, a high-voltage pulse penetrating into the electronic transformer via the mains terminal can have a destructive effect by charging the starting capacitor and therefore triggering a starting operation. This undesired starting operation causes no damage whenever the switch driven by the starting circuit is instantaneously switched on during the running natural oscillation. However, in the case of electronic transformers according to the prior art it is precisely when it is switched on instantaneously that the starting capacitor is discharged synchronously therewith and an undesired starting operation is suppressed. According to the invention, a starting operation is always, or even suppressed when the switch of the half-bridge is driven which is not driven during a starting operation. Consequently, an undesired starting operation triggered by a high-voltage pulse can drive only the switch of the half-bridge which is in any case instantaneously switched on. This avoids destruction of the electronic transformer by the above named cross currents.
The idea of the invention is realized by virtue of the fact that the electronic discharging switch included in the blocking device for discharging the starting capacitor is driven whenever, or even whenever precisely that switch of the half-bridge is driven which is not driven during a starting operation. This can be accomplished in a plurality of ways.
Self-excited electronic transformers often include a drive transformer which uses secondary windings to feed back an output variable of the half-bridge inverter onto the control electrodes of the switches for the half-bridge, the result being a self-excited oscillation. According to the invention, a further secondary winding is applied to this drive transformer. A drive signal for the electronic discharging switch included in the blocking device is generated via an electric network from the signal which is tapped at this further secondary winding. The sense of the further secondary winding is selected according to the invention such that this discharging switch is always driven when that switch of the half-bridge is driven which is not driven in the event of a starting operation. The different refinements for the said network are discussed in the description of the drawings.
A further embodiment of the idea of the invention consists in that a drive signal for the electronic discharging switch included in the blocking device is always generated when a high-voltage pulse occurs at the mains terminals. This can be achieved by virtue of the fact that the current activates the blocking device by means of a switching means which becomes conductive in the event of an overvoltage occurring at a voltage supply of the electronic transformer. An exemplary embodiment for this purpose is described in FIG. 7.